A navigation device displays a current position of a subject vehicle on a road map indicated by map data based on an absolute position detected. This absolute position is detected by using both (i) absolute measurement data based on a satellite navigation method using a GPS sensor and (ii) relative measurement data based on a dead-reckoning navigation method using a gyro sensor, a speed sensor, or the like. Here, if a road map indicated by the map data has an error against the real road, an absolute position of the vehicle does not match with the road map indicated by the map data.
To that end, a navigation device performs a map-matching process for amending a vehicle position to match with a road map indicated by map data, and obtains a vehicle position based on the road map. The navigation device is thus required to display a vehicle position to match with a road map. In other words, even when a real road is significantly different from a road map (or a road on a road map) indicated by map data, a vehicle position is required to be displayed constantly to match with the road map.
In recent years, a navigation device has developed to cooperate with a vehicle control. For instance, a radiation direction of a headlight of a vehicle is controlled based on information on a curve obtained from map data. Controlling the vehicle to follow a road state based on the map data in the navigation device naturally requires an improvement in accuracy of the map data. To that end, a system is proposed which includes an information center for storing and maintaining road data (see Patent document 1).                Patent document 1: JP-3496479 B2 (U.S. Pat No. 6,154,152)        
However, this system is intended to add new road data different from the road data already stored in the information center, and not to update the road data already stored in the information center. Therefore, accuracy in the already stored road data cannot be increased.